Vascular smooth muscle cell (VSMC) proliferation and migration are the major causes of coronary artery in- stent restenosis and accelerated arteriopathy following cardiac transplantation. How VSMC proliferation, migration, and consequent restenosis can be prevented in vivo remains a subject of extensive research in the last decade. Our exciting preliminary data suggest that pharmacological or genetic activation of AMP-activated protein kinase (AMPK) is able to suppress VSMC proliferation and neointimal hyperplasia in vivo. Fluorescence-activated cell sorting (FACS) analysis of VSMC from mice revealed that loss of AMPKa2 increased VSMC transition from G1 to S phase. Consistent with this finding, the cell cycle inhibitor, p27Kip1 (p27), was dramatically down-regulated in AMPKa2-knock out (KO) mouse VSMC but not AMPKa1-KO VSMC. In addition, we found that p27Kip1 deregulation was not due to p27Kip1 mRNA level but due to high Skp2 expression, a subunit of ubiquitin E3 ligase through the STAT binding in the Skp2 promoter. Mechanistically, we found that the S-phase kinase-associated protein 2 (Skp2), an E3 ubiquitin ligase for p27, was elevated in AMPKa2-KO VSMC and was responsible for increased degradation of p27. The most conclusive evidence for AMPK-dependent inhibition of VSMC proliferation and consequent restenosis was that wire injury-induced neointima hyperplasia in the carotid artery was significantly greater in AMPKa2-KO mice than in either AMPKa1-KO or wild type (WT) animals. Thus, the central hypothesis of this application is that loss of AMPKa2 increases Skp2, an E3 ligase for p27, and Skp2-mediated degradation of p27 to produce aberrant VSMC proliferation and migration, critical events in the development of neointimal hyperplasia and restenosis. This hypothesis will be tested in three specific aims: Aim #1 is to establish the central roles of p27 in aberrant VSMC proliferation and migration caused by AMPKa2 inactivation. Aim #2 is to determine if and how Skp2 up-regulation by AMPKa2 deletion causes p27 degradation and enhanced cell proliferation and migration in AMPKa2-KO VSMC. In the last Aim, we will establish a central role for Skp2 and p27 in neointimal hyperplasia in vivo. A combination of in vitro and in vivo techniques, gain-/loss-of-function, and pharmacologic/genetic approaches will used to accomplish the study objectives. The completion of this project will provide novel insights into whether AMPK, p27, and Skp2, are potential therapeutic targets for countering vascular damage associated with common diseases including diabetes, restenosis, atherosclerosis, and cancer.